The present invention relates to an active material used for a non-aqueous electrolyte secondary battery and a manufacturing method of the active material.
Lithium ion batteries used as primary power sources for mobile communication devices and portable electronic devices are characterized by high electromotive force and high energy density.
Examples of positive electrode active materials used for such lithium ion batteries include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), manganese spinel (LiMn2O4), mixtures thereof, etc. These active materials have a voltage of not less than 4V relative to that of lithium. On the other hand, a carbon material is usually used in a negative electrode, which is combined with a positive electrode including the above-mentioned positive electrode active material to give a 4V level lithium ion battery.
Various technologies using a titanium oxide as a negative electrode active material have also been reported.
For example, Japanese Laid-Open Patent Publication No. Hei 06-275263 discloses a battery in which a titanium oxide is used for a negative electrode and a spinel-type manganese oxide or LiCoO2 is used for a positive electrode, and Japanese Laid-Open Patent Publication No. Hei 07-320784 discloses a non-aqueous electrolyte lithium secondary battery comprising a negative electrode in which a spinel-type lithium-titanium oxide (Li4/3Ti5/3O4) is used as an active material, a positive electrode in which Li2MnO3 or LiMnO2 is used as an active material, and an electrolyte.
Also, Japanese Laid-Open Patent Publication No. Hei 11-283624 discloses a lithium secondary battery in which Li2Ti3O7 is used for a negative electrode, and Japanese Laid-Open Patent Publication No. 2000-302547 discloses a manufacturing method of a negative electrode in which a preferable titanium oxide is used.
Unlike graphite used for a negative electrode, the titanium oxide useful for the active material disclosed in the above patent publication documents is low in electron conductivity. Also, even LiCoO2 which comparatively has a better conductivity, the resistivity is about 1×104 Ωcm.
Therefore, materials with high conductivity such as acetylene black and graphite are generally used as a conductive auxiliary agent, when the titanium oxide such as the above is used for batteries.
On the other hand, Japanese Laid-Open Patent Publication No. Hei 11-102705 discloses a usage of a compound including oxygen and nitrogen, and at least one of silicon, germanium, and tin. Such active material is represented by a general formula: MxNyOz, wherein, M is at least one element selected from the group consisting of Si, Ge, and Sn, and x, y, and z are respectively 1.4<x<2.1, 1.4<y<2.1, and 0.9<z<1.6.
The object of the Japanese Laid-Open Patent Publication No. Hei 11-102705 is to provide an active material for a negative electrode with high capacity. In the active material of the Japanese Laid-Open Patent Publication No. Hei 11-102705, matrix-like quasi planes formed of chair-type six-membered rings comprising silicon and nitrogen spreads out, and in between these planes (between these layers), silicon—oxygen—silicon binds exist to crosslink, thereby creating one dimensional tunnel-like parts. The tunnel like parts become dope and de-dope site of the battery, and the negative electrode material to be obtained exerts a great capacity.
Therefore, in the negative electrode active material shown in Examples of Japanese Laid-Open Patent Publication No. Hei 11-102705, a crystal structure as a matrix comprising Si2N2O, Ge2N2O, or Sn2N2O that are nitrogen oxide having N is important. However, Japanese Laid-Open Patent Publication No. Hei 11-102705 have no disclosure or implication on electron conductivities of the above negative electrode active material, and in its Examples, a carbon material is simply mixed as the conductive auxiliary agent.
As described above, when an oxide such as a titanium oxide is used in an active material for a negative electrode, highly conductive materials such as acetylene black and graphite also have to be used in combination as a conductive auxiliary agent, due to low electron conductivity of the oxide. However, since these conductive auxiliary agents are not power generating elements, battery capacity per unit volume is caused to decrease significantly.
Further, a titanium oxide TiO2, for example, is an insulating material with a resistivity of about 1×1014 Ωcm. When such material with poor electron conductivity is used as an active material, simply mixing in a conductive auxiliary agent is not sufficient. Some techniques such as increasing specific surface areas of active material particles, or coating surfaces of active material particles with a graphite material are necessary for the active material to work. However, granulation by grinding will lead to a decrease in charging amount, a further decrease in capacity, and an increase in costs for the manufacturing process.